The primary modes of altering myocardial contractility on a beat-to-beat basis involve both the modulation of the myoplasmic Ca2+ ([Ca2+]i) availability at the level of the myofilaments (MF), and changes in the Ca2+ response of the MF, especially via length-dependent activation. Certain more slowly developing covalent modifications of MF have been recognized (such as thick or thin filament phosphorylation). We have found, for example, that the [Ca2+]i "history", via Ca2+-calmodulin-dependent myosin light chain kinase (MLCK) activity, regulates contractility in isolated adult rat heart cells apparently via myosin light chain (MLC2) phosphorylation. Certain vasoactive substances elaborated by the vascular endothelium and endocardium, such as endothelin (ET-1), have recently been recognized to modulate myocardial contractility, although the mechanisms remain controversial. We have found that ET-1 induces a time-dependent increase in Ca2+-MF sensitivity in as isolated adult rat cardiac myocyte model, effects persisting (after washout) on the order of that of the time of ET-1 exposure. On the other hand, these MF sensitivity changes can be rapidly reversed by the nonspecific phosphatase 2,3-butanedione monoxime (BDM). This ET-1 effect is mimicked by the phorbol ester PMA (without distinct, additive effects by the combination of ET-1 and PMA) and by the protein phosphatase inhibitor calyculin A (also rapidly reversed by BDM), and was attenuated by the PKC inhibitor calphostin C. In contrast, the ET- effect was not affected by the MLCK inhibitor KT5926, the cGMP-dependent protein kinase inhibitor KT5823, nor by the Ca2+/calmodulin kinase inhibito KN-62. These data support the hypothesis that activation of the PKC signalling pathway, and probably the resulting MF phosphorylation and sensitization, is at least partially responsible for the positive inotropic action ET-1.